The present invention relates to a synchrotron type accelerator and a medical treatment system employing the same.
A conventional synchrotron dedicated to medical use is described in an article of Proc. of the 8-th Symposium on Accelerator Science and Technology, 1991, Saitama, Japan, p.414 (FIG. 3).
A conventional synchrotron type acceleration is shown in FIG. 5. When ejecting a charged particle beam, a multipolar electromagnet 11 for resonance excitation is excited so that the resonance of the betatron oscillations of the charged particle beam causes the amplitude of the betatron oscillations to increase. Then, the charged particle beam having the increased amplitude of the betatron oscillations is ejected to a medical treatment room 1010 through an electrostatic deflector 101 and a deflecting electromagnet 102 which are arranged in a straight section for use in ejecting therethrough the charged particle beam.
The electrostatic deflector 101 is shown in FIG. 6. A high voltage is applied across electrodes 1031 and 1032 by a power source 131 to generate a horizontal electric field. The charged particle beam which enters into a space defined between the electrodes 1031 and 1032 due to increase of the amplitude of the betatron oscillations is deflected by the horizontal electric field to enter into an ejected beam transporting system to be ejected therethrough. On the other hand, the charged particles which did not enter into the space between the electrodes 1031 and 1032 circulate along the accelerator without being ejected. Then, the amplitude of the betatron oscillations of such charged particles are again increased and then are ejected through the ejected beam transporting system just after having entered into the space between the electrodes.
The deflecting electromagnet 102 for the beam ejection is shown in FIG. 7. At the time when a current is supplied from a power source 131 to coils 1041 and 1042, the vertical magnetic field is generated between the coils 1041 and 1042. The charged particle beam is further deflected by this vertical magnetic field to be ejected to the ejected beam transporting system.
In the electrostatic deflector 101, the charged particle beam which has collided with the end face of the electrode 1031 is lost. Therefore, in order to minimize the beam loss, the electrode 1031 needs to be thin. In the case of the deflector of the electrostatic type, however, it is difficult to increase the electric field strength up to a level equal to or larger than 90 kV/cm, and hence it is impossible to deflect sufficiently the charged particle beam.
In the deflecting electromagnet 102 for beam ejection, in order to suppress the heating of the coils, the coil 1041 is thickened. However, since the charged particle beam collides with the coil 1041, this results in the beam loss being increased.
In the conventional synchrotron, as shown in FIG. 5, in which the electrostatic deflector 101 is provided upstream, and the deflecting electromagnet 102 for beam ejection is provided downstream, both the electrostatic deflector 101 and the deflecting electromagnet 102 for beam ejection have to be long enough in the charged particle beam travelling direction in order to deflect sufficiently the charged particle beam in the electrostatic deflector 101 and also to reduce the beam loss in the deflecting electromagnet 102 for beam ejection. As a result, the dimensions of the synchrotron becomes large.